The present invention relates to an apparatus for optically measuring the distance from a vehicle to objects such as obstacles lying ahead thereof as well as other vehicles which are travelling ahead of the vehicle or approaching it from behind.
Some examples of an obstacle detecting apparatus are disclosed in Japanese Patent Laid-Open 55-15337 and Japanese Utility Model Laid-Open 1-12221. These apparatuses project ultrasonic waves or electromagnetic waves in a specified forward or backward direction to be detected and receive reflected waves from an object or obstacle for the purpose of determining the presence or absence of an obstacle as well as measuring the distance thereto.
Examples of an optical distance detecting apparatus utilizing image sensors are known from Japanese Patent Publication Nos. 63-38085 and 63-46363. As illustrated in FIG. 8, the apparatuses disclosed therein commonly have a pair of first and second parallel optical systems having two convex lenses 101, 102 disposed in a horizontally aligned relation at a prescribed distance L away from each other. A pair of separate image sensors 103, 104 are horizontally disposed at focal points of the lenses 101, 102 at a distance f from the locations of corresponding lenses 101, 102, respectively, for generating respective image signals to a common signal processor 120. The signal processor 120 successively shifts the image signals from the image sensors 103, 104 and electrically superposes them one over the other, so that the distance R from the lenses 101, 102 to an object 121 is calculated based on the principle of triangulation using the following formula: EQU R=(f.times.L)/d
where d is a shift distance by which the image signals are moved to obtain the best match.
In addition, Japanese Patent Publication No. 63-18122 discloses an obstacle detecting apparatus for a vehicle which is provided with a plurality of light emitting elements or light transmitters each projecting a beam of light toward objects to be detected for the purpose of improving the contrast of the image of an object sensed by the image sensors in the event that no good image contrast is obtained.
With the distance and/or obstacle detecting apparatuses as described above, it is possible to detect the presence or absence of an object lying in the direction in which ultrasonic or electromagnetic waves or light are projected, but it is difficult to precisely determine whether the object thus detected is an obstacle to the travel of the vehicle. For example, when a vehicle is travelling on a curved road, the obstacle detecting apparatus mounted thereon can misidentify a guide rail, which is set up along one side or shoulder of the curved road and lies ahead of the cornering vehicle, as an obstacle. In addition, in order to exactly determine the location and direction of the object detected with respect to the moving vehicle, many ultrasonic or electromagnetic wave transmitters and receivers are required, thus resulting in a great increase in size and costs of manufacture of the overall apparatus.
Moreover, if there are many objects within the field of view of the image sensors, it is unclear to which object the distance from the subject vehicle is being detected. In particular, let us assume that the distance to an object in the form of a preceding vehicle is detected by the above-described distance detecting apparatus mounted on the subject vehicle during the travel thereof. In this case, if another vehicle travelling on one of plural lanes of a road adjacent the one on which the subject vehicle is travelling comes into the field of view of the image sensors, it is not clear at all or the driver cannot tell to which vehicle (i.e., travelling on the same lane or on an adjacent lane) the distance is detected.
Furthermore, in cases where the brightness or magnitude of external light is insufficient for proper distance detection by image sensors such as when the vehicle travels in a tunnel or in dark weather such as in the evening, twilight, etc., it is necessary to use a light emitting element which projects a beam of visible light toward a preceding vehicle. In this case, however, there is a fear that the passengers including the driver in the preceding vehicle are dazzled by the light from the light emitting element, making it difficult for the driver to properly maneuver his or her vehicle in a safe manner.